Solubilizing Metal–Organic Frameworks for an In Situ IR-SEC Study of a CO2 Reduction Catalyst

Metal–organic frameworks (MOFs) are typically assembled by bridging metal centers with organic linkers for various applications, including providing robust support for heterogeneous catalysts for CO2 reduction. In this study, we have demonstrated the solubilization of a MOF tethered to a CO2-reducing electrocatalyst and studied its fundamental electrochemistry in THF solvent using infrared spectroelectrochemistry (IR-SEC). The fundamental electrochemical properties of this immobilized catalyst were compared to that of its homogeneous counterpart. This approach provides a foundation for future experimental studies to bridge the gap between homogeneous and heterogeneous electrocatalysis.


■ INTRODUCTION
−3 However, the discovery and optimization of these catalysts rely mostly on high-throughput screening, and a lack of mechanistic understanding typically hinders the rational design of highly selective heterogeneous catalysts. 2,3By comparison, molecular-engineered homogeneous catalysts are mechanistically well-understood. 2 To bridge the gap, molecular complexes are typically immobilized onto a solid surface to provide mechanistic insight into the activity of the resulting species, aiding in the development of catalysts that are both efficient and selective. 2,4−16 In some cases, anchoring these complexes to a MOF support results in better catalytic activity than the molecular system. 10,17,18−23 These MOFs also offer the possibility of CO 2 absorption, which may enhance catalytic activity by increasing the effective substrate concentration at active sites. 16,22,24wever, because MOFs are not soluble and therefore not accessible to traditional condensed-phase transmission spectroscopies, it is difficult to identify, characterize, and study the reactivity of the relevant intermediate species generated in these reactions.This limitation makes it challenging to obtain mechanistic information about the reactions and impedes the application of a rational design process to develop more efficient catalysts.Recently, we established that tethering polar and nonpolar oligomers to the MOF surface generates MOF nanoparticles that are soluble in both polar and nonpolar solvents and can function as supports for transition metal catalysts that are observable by transmission IR spectroscopy. 25his discovery opens the way to investigate the operational mechanism of a number of catalytic processes mediated by MOF-immobilized transition metal catalysts using traditional spectroscopic methods to monitor the reaction progress and identify relevant intermediates.
−30 In the present work, we extend these studies to investigate the analogous electrochemistry of the solubilized MOF-supported (bipy)Re(CO) 3 Cl catalyst.Solubilization of the MOF particles in THF was achieved by grafting polyisobutylene (PIB) oligomers onto the MOF surface, thereby enabling in situ acquisition of solution-state IR spectra under applied potentials.The basic electrochemistry of this catalytic system was investigated and found to mimic the behavior of the homogeneous molecular system with some differences.Such experiments were previously inaccessible due to the insolubility of MOFs and therefore serve as a first example of the application of this technique toward an insolution study of MOF-immobilized transition-metal complexes for CO 2 reduction.It is hoped that this proof-of-concept study can be further leveraged to gain mechanistic insight into reactions catalyzed by single-molecule heterogeneous complexes tethered to MOF surfaces.

■ RESULTS AND DISCUSSION
Recently, we reported a method to solubilize an azidefunctionalized MOF (UiO66-N 3 ) with PIB.An alkyne bipyridine Re carbonyl complex was tethered to the MOF surface with catalytic click chemistry. 25Simplifying this approach, in this study, the UiO66-NH 2 MOF was functionalized with aldehyde derivatives of bipyridine by an uncatalyzed condensation reaction under sonication and reflux conditions. 15Under these conditions, a toluene suspension of this MOF in the presence of M(CO) 5 X [M = Re, Mn, X = Cl and Br] resulted in the immobilization of the (bipy)M(CO) 3 X complex on the MOF surface (Figure 1A).Subsequently, phosphonic-acid-functionalized PIB was grafted onto the resulting MOF by utilizing its metal-oxo nodes (Figure 1B, see SI for details).
The presence of metal carbonyl complexes on the MOF surface was confirmed using a variety of techniques as reported previously, including IR spectroscopy, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energydispersive X-ray spectroscopy (EDX) mapping, and TGA (Figures S2−6). 25The EDX analysis showed a uniform distribution of the immobilized complexes throughout the surface of these MOFs (Figures S4−5).The results of PXRD and SEM analyses verified the conservation of the crystalline structure of the materials during the condensation reaction (Figures S2−3).A solid-state IR spectrum of both UiO66-(bipy)Re(CO) 3 Cl (MOF-a) and (PIB)MOF-a showed peaks at 2031 cm −1 and a broad peak at 1933 cm −1 (Figure S7).In the solution state (Figure S9), the corresponding peaks for (PIB)MOF-a were observed at 2023 and 1899 cm −1 (broad).These absorbances were assigned to the Re-bound terminal CO groups, and their positions are comparable to those observed for the analogous molecular complex (Figure S8). 30 Similarly, the solid-state IR spectrum of UiO66-(bipy)Mn-(CO) 3 Br (MOF-b) displayed peaks at 2031 cm −1 and a broad peak at 1933 cm −1 (Figure S10).Because of its greater stability, IR-SEC experiments were conducted only on the rhenium system, (PIB)MOF-a.
To compare the condensed-phase electrochemistry of the MOF-immobilized and molecular rhenium carbonyl complexes, initial investigations focused on (bipy)Re(CO) 3 Cl (1).This complex was subjected to an IR-SEC study in THF solvent to identify and establish the spectral features of important electrocatalytic intermediates.The observed spectral changes were consistent with those reported in the literature and are shown in Figure 2A.Consistent with the work of Kubarych and co-workers, the intermediates detected are shown in Figure 2B. 28In the present study, as the voltage was scanned at the rate of 2 mV/s, covering a range from 0 to −2.5 V (vs the Fc/Fc + couple), the characteristic CO peaks of the parent complex 1 in THF (2023, 1918, and 1896 cm −1 ) decreased in intensity, while growth of peaks assigned to the first reduced species [(bipy)Re(CO) 3 Cl] − * (1a) began to appear at 1996, 1883, and 1868 cm −1 .As the voltage was further decreased, signature peaks of the dimer species (bipy) 2 Re 2 (CO) 6 (1b) appeared at 1986 cm −1 , 1950 cm −1 , 1888 cm −1 , and 1854 cm −1 followed by the generation of the catalytically active second reduced species [(bipy)Re(CO) 3 ] − (1c) with CO stretching absorbances at 1947 and 1843 cm −1 .
The fundamental electrochemistry of (PIB)MOF-a was studied in a manner similar to that for complex 1.Thus, (PIB)MOF-a was dissolved in a THF solution containing 0.1 M TBAPF 6 as the electrolyte and the voltage scanned at a rate of 2 mV/s covering the range from 0 to −2.0 V.The cyclic voltammograms (CVs) of 1 and (PIB)MOF-a are shown in Figure 3. Consistent with the literature, the first and second reduction waves for 1 are observed at −1.38 V and −1.88 V, respectively. 27The reduction potentials for (PIB)MOF-a are shifted slightly from those of 1, appearing at −1.50 V and −1.78 V, respectively, and suggest that immobilization of (bipy)Re(CO) 3 Cl on the MOF surface does not significantly affect the reduction potentials of this complex.Unlike 1, the first reduction at −1.50 V does not appear to be reversible in the case of (PIB)MOF-a which may be due to decreased solubility of the reduced complexes.
As shown in Figure 4, the IR-SEC spectra for (PIB)MOF-a are quite different from those obtained upon electrolysis of 1.
In general, the CO stretching absorbances of the immobilized rhenium carbonyl complex in (PIB)MOF-a are broad, most likely due to differences in the local environment on the MOF surface.As the voltage is scanned negative, the parent peaks decrease in intensity, and four new peaks are observed.Similar to 1, the broad CO stretching band from 1888 to 1850 cm −1 most likely includes the E symmetry stretches of several electrochemically generated species with a fac-[Re(CO) 3 ] geometry, and the lack of spectral resolution in this region does not assist in determining the identity of these intermediates.However, the companion A 1 stretches observed in the high cm −1 region are somewhat better resolved and are more amenable to interpretation.Peaks at 1996, 2008, and 2015 cm −1 (overlapped) are observed corresponding to the generation of three possible intermediates.
In identifying the species associated with these peaks, we have relied on the reported values of the CO stretching cm −1 of intermediates observed upon electrolysis of several (αdiimine)Re(CO) 3 X complexes. 30,36This comparison is justified because the position of the CO stretching bands of 1 (2023, 1918, and 1896 cm −1 ) and (PIB)MOF-a (2023 and 1899 cm −1 (broad)) are similar, suggesting that tethering 1 to the MOF does not dramatically influence the electron density on the Re center.Thus, the peak at 1996 cm  30,36 Notably, the THF radical species was mostly observed in complexes in which dimerization was not detected, presumably due to the steric bulk of the diimine ligand (which would certainly be the case in the present study).Finally, as previously reported for the molecular system, the cationic species (PIB)MOF-e may be generated by oxidation of the radical species (PIB)MOF-d by residual oxygen in solution. 28The proposed reaction scheme upon electrolysis of (PIB)MOF-a is shown in Figure 5.
In dramatic contrast to 1, evidence in the IR-SEC spectra was not obtained for the formation of the analogous dimer or   the second reduced species (PIB)MOF-f in the electrolysis of (PIB)MOF-a.The lack of dimerization can be easily justified since the large steric profile of the MOF would inhibit this reaction, as in the case of other molecular (α-diimine)Re-(CO) 3 X complexes. 2,26,30,36The failure to observe a CO stretching absorbance consistent with the second reduced species in the MOF system (expected at ∼1950 cm −1 ), even though the CV confirms a second reduction step (Figure 3), is harder to explain.Perhaps an insufficient amount of (PIB)-MOF-f is formed to be detected by IR spectroscopy, or it is insoluble in THF and is therefore unobservable in solution.
A few preliminary experiments were conducted to determine whether (PIB)MOF-a was capable of reducing CO 2 like its molecular counterpart.The CV of a CO 2 -saturated THF solution of complex (PIB)MOF-a shows a slight increase in current at both reduction potentials, which is indicative of catalytic activity.The increase in current at −1.77 V indicates the formation of the second reduced species, which is responsible for CO 2 reduction (Figure 6).
However, because it is challenging to calculate the concentration of the rhenium complex in the MOF system, a direct comparison of catalytic efficiency between 1 and (PIB)MOF-a remains elusive.Further investigations are ongoing to identify and study the reactivity of possible CO 2 reduction intermediates.Different MOF motifs (e.g., replacement of Zr by Hf) are also being explored.

■ CONCLUSION
This work highlights a new method for solubilizing heterogeneous electrocatalyst-immobilized MOFs in organic solvents for in situ spectroscopic studies.We have shown that condensed-phase IR-SEC can be used to identify electrochemically generated intermediates that have previously been difficult to access.It is hoped that this work can be leveraged to perform in situ mechanistic studies of single-molecule heterogeneous electrocatalysts using solution-phase analytic techniques.
■ ASSOCIATED CONTENT * sı Supporting Information

Figure 2 .
Figure 2. (A) Difference FTIR spectra obtained upon electrolysis of 1 over a voltage range of −1.25 to −2.50 V at a scan rate of 100 mV/s.The reference IR spectrum is of the solution before application of a potential difference.(B) Intermediates observed during the electrolysis of 1 in THF (adapted from ref 9 c).While this experiment was conducted in the absence of CO 2 , intermediate 1c has been identified previously as the catalytically active species in CO 2 reduction.

Figure 3 .
Figure 3. CV of 1 and (PIB)MOF-a in THF solvent.All measurements were recorded at 100 mV/s, under argon in THF with 0.1 M TBAPF 6 supporting electrolyte (WE = glassy carbon, CE = Pt wire, RE = Ag/AgCl, and Fc +/0 as internal reference standard).

Figure 4 .
Figure 4. Observed difference IR-SEC spectra obtained upon electrolysis of (PIB)MOF-a in THF solvent at scan rate of 2 mV/s.The reference spectrum is of the solution before application of a potential difference.

Figure 5 .
Figure 5. Proposed initial mechanism of the (PIB)MOF-a electrocatalytic reduction cycle in THF.While CO 2 was not added to the solution, it is shown in this scheme to complete the cycle.

Figure 6 .
Figure 6.CV of (PIB)MOF-a in THF solution in the only Ar, CO 2 , and CO2 and water.Water acts as proton source to complete the reduction of CO 2 .All measurements were recorded at 100 mV/s, under argon in THF with 0.1 M TBAPF 6 supporting electrolyte (WE = glassy carbon, CE = Pt-wire, RE = Ag/AgCl, and Fc +/0 as internal reference standard).